A common mode choke coil including a laminated-type coil includes a multilayer body having a laminated structure with a plurality of laminated insulation layers, and a coil is provided within the multilayer body. The coil includes a plurality of spiral-shaped coil conductors. Each of the plurality of coil conductors has an inner circumferential side end portion located relatively near a central area of the insulation layers and an outer circumferential side end portion located relatively near an outer edge of the insulation layers, with an inner circumferential side via hole conductor being connected to the inner circumferential side end portion and an outer circumferential side via hole conductor being connected to the outer circumferential side end portion. To create a portion in the coil having mutually opposite winding directions, the plurality of coil conductors are connected in series by alternately using the inner circumferential side via hole conductors and the outer circumferential side via hole conductors so that the inner circumferential side end portions are connected to each other by the inner circumferential side via hole conductors and the outer circumferential end portions are next connected to each other by the outer circumferential side via hole conductors.
Japanese Unexamined Patent Application Publication No. 2003-68528 and Japanese Unexamined Patent Application Publication No. 2001-44033, for example, disclose common mode choke coils of interest in the context of this disclosure.
Japanese Unexamined Patent Application Publication No. 2003-68528 and Japanese Unexamined Patent Application Publication No. 2001-44033 disclose forming a primary coil by forming a spiral-shaped coil conductor on an insulation layer, laminating a plurality of these insulation layers together, and connecting the plurality of coil conductors in series through via hole conductors, and forming a secondary coil by forming a spiral-shaped coil conductor on an insulation layer, laminating a plurality of these insulation layers together, and connecting the plurality of coil conductors in series through via hole conductors.
The common mode choke coil disclosed in Japanese Unexamined Patent Application Publication No. 2003-68528 in particular has a structure in which a portion where only the plurality of insulation layers for the primary coil are laminated and a portion in which only the plurality of insulation layers for the secondary coil are laminated are disposed so as to be isolated from each other.
On the other hand, the common mode choke coil disclosed in Japanese Unexamined Patent Application Publication No. 2001-44033 has a structure in which the insulation layers for the primary coil and the insulation layers for the secondary coil are laminated in an alternating manner, or in other words, a structure in which coil conductors for the primary coil and coil conductors for the secondary coil are laminated in an alternating manner.
According to the common mode choke coil disclosed in Japanese Unexamined Patent Application Publication No. 2003-68528, the primary coil and the secondary coil are positioned so as to be isolated from each other, resulting in a weak coupling between the primary coil and the secondary coil. There is thus a problem that desired characteristics are difficult to achieve.
As opposed to this, the common mode choke coil disclosed in Japanese Unexamined Patent Application Publication No. 2001-44033 has a structure in which the coil conductors for the primary coil and the coil conductors for the secondary coil are laminated in an alternating manner, and thus a relatively strong coupling can be achieved between the primary coil and the secondary coil. However, in the case where this type of alternating laminated structure is employed, a via hole conductor that connects coil conductors for one of the coils will unavoidably pass through two insulation layers that form a boundary surface along which a coil conductor for the other coil extends, which may cause problems such as those described below.
FIG. 7 illustrates a cross-sectional view of a part of a common mode choke coil employing an alternating laminated structure, and specifically illustrates a portion in which are located two adjacent coil conductors 1 and 2 for a first coil and a via hole conductor 3 that connects the coil conductors 1 and 2, along with several insulation layers 4 to 8 and a coil conductor 9 for a second coil. Although not illustrated in FIG. 7, the coil conductor for the second coil extends at least along a boundary surface between the insulation layers 5 and 6.
As illustrated in FIG. 7, a via pad 3a is formed at each boundary surface position between the insulation layers 5 to 7 so as to extend outward around the via hole conductor 3. Although formed at the same time as when a conductive paste for the via hole conductor 3 is applied, the via pad 3a contributes to an increase in the reliability of the connection between the via hole conductor 3 and the coil conductors 1 and 2, as well as an increase in the reliability of the connection of the via hole conductor 3 at the boundary surface between the insulation layers 5 to 7, even if, for example, skew in the lamination of the insulation layers 4 to 7 has arisen. As such, the via pad 3a normally tends to have a greater thickness than the thicknesses of the coil conductors 1 and 2.
In the case where an alternating laminated structure is employed, the via hole conductor 3 that connects the coil conductors 1 and 2 to each other is provided so as to pass through the two insulation layers 5 and 6 as mentioned above. Three via pads 3a overlap in the lamination direction as a result. This in turn results in a greater length of the via hole conductor 3 in the axis line direction than in the case where a via hole conductor passes through one insulation layer only, which means that a greater amount of conductive material provided for the via hole conductor 3 and the via pad 3a is present near the via hole conductor 3.
In the case where the insulation layers 4 to 8 are formed from a glass ceramic material, for example, a firing process is carried out during the manufacture of the common mode choke coil. In the firing process, the conductive material for the via hole conductor 3 and the via pad 3a normally diffuses into the insulation material provided for the insulation layers 4 to 8. As described above, there is a greater amount of conductive material in the structure illustrated in FIG. 7 than in the case where the via hole conductor passes through a single insulation layer only, and thus the amount of diffused conductive material is greater in the structure illustrated in FIG. 7.
Meanwhile, in the process for manufacturing the common mode choke coil, a process for pressing the insulation layers 4 to 8 in the lamination direction is carried out in a stage before the firing in order to increase the tightness of the lamination. The conductive material used for the via hole conductor 3 and the via pad 3a is less susceptible to compression deformation due to the pressing process than the insulation material used for the insulation layers 4 to 8. As such, the insulation layer 7, for example, is compressed more at areas where the via hole conductor 3 and the via pad 3a are located, and a thickness T of the insulation layer 7 at these areas becomes significantly lower than the original thickness of the insulation layer 7. The same drop in thickness can occur in the insulation layer 4 as well.
The stated conductive material diffusion, drop in thickness of the insulation layers 4 and 7, and so on become factors leading to a drop in the breakdown voltage reliability of the common mode choke coil. In the case where a conductor that can generate a potential difference between itself and the via hole conductor 3 and the via pad 3a, such as the coil conductor 9 for the second coil, for example, is formed on a top surface side of the insulation layer 7 as illustrated in FIG. 7 so as to be located on a line extending from an axis line of the via hole conductor 3, the breakdown voltage reliability between the coil conductor 9 and the via pad 3a becomes a concern. Purely from the standpoint of conductive material diffusion, the same breakdown voltage reliability problem can arise in the case where an outer terminal electrode (not shown) that can generate a potential difference between itself and the via hole conductor 3 and the via pad 3a is located near the via hole conductor 3 or the via pad 3a. 
The via hole conductor 3 illustrated in FIG. 7 can be the inner circumferential side via hole conductor that connects the inner circumferential side end portions of the coil conductors 1 and 2 to each other or the outer circumferential side via hole conductor that connects the outer circumferential side end portions of the coil conductors 1 and 2 to each other. It is particularly difficult to avoid the aforementioned breakdown voltage reliability problem in the case where the via hole conductor 3 is the outer circumferential side via hole conductor. A reason for this will be described next.
First, assume that insulation layers 11 to 15, illustrated in FIG. 8, are laminated in that order from the bottom to form the multilayer body of the common mode choke coil.
A spiral-shaped coil conductor 16 for a primary coil is formed on the insulation layer 11, a spiral-shaped coil conductor 17 for a secondary coil is formed on the insulation layer 12, a spiral-shaped coil conductor 18 for the primary coil is formed on the insulation layer 13, a spiral-shaped coil conductor 19 for the secondary coil is formed on the insulation layer 14, and a spiral-shaped coil conductor 20 for the primary coil is formed on the insulation layer 15.
In FIG. 8, an inner circumferential side end portion of the coil conductor 16 on the insulation layer 11 and an inner circumferential side end portion of the coil conductor 18 on the insulation layer 13 are connected to each other by an inner circumferential side via hole conductor 21 as indicated by a dashed line. An outer circumferential side end portion of the coil conductor 18 on the insulation layer 13 and an outer circumferential side end portion of the coil conductor 20 on the insulation layer 15 are connected to each other by an outer circumferential side via hole conductor 22. On the other hand, an outer circumferential side end portion of the coil conductor 17 on the insulation layer 12 and an outer circumferential side end portion of the coil conductor 19 on the insulation layer 14 are connected to each other by an outer circumferential side via hole conductor 23. The stated inner circumferential side via hole conductor 21 passes through the two insulation layers 12 and 13, the outer circumferential side via hole conductor 22 passes through the two insulation layers 14 and 15, and the outer circumferential side via hole conductor 23 passes through the two insulation layers 13 and 14.
Such connections are also realized in coil conductors that are not illustrated. For example, the inner circumferential side end portion of the coil conductor 17 on the insulation layer 12 and the inner circumferential side end portion of the coil conductor on the insulation layer laminated to the bottom of the insulation layer 11 are connected by an inner circumferential side via hole conductor 24, and the inner circumferential side end portion of the coil conductor 19 on the insulation layer 14 and the inner circumferential side end portion of the coil conductor on the insulation layer laminated to the top of the insulation layer 15 are connected by an inner circumferential side via hole conductor 25.
Consider the inner circumferential side via hole conductor 21 and the outer circumferential side via hole conductor 23 as representative examples. A positional relationship between the inner circumferential side via hole conductor 21 and the coil conductor 19 is similar to a positional relationship between the via hole conductor 3 and the coil conductor 9 illustrated in FIG. 7. Likewise, a positional relationship between the outer circumferential side via hole conductor 23 and the coil conductor 20 or the coil conductor 16 is similar to the positional relationship between the via hole conductor 3 and the coil conductor 9 illustrated in FIG. 7. As such, the aforementioned breakdown voltage reliability problem can arise in either case.
However, with respect to the positional relationship between the inner circumferential side via hole conductor 21 and the coil conductor 19 mentioned first, it is relatively easy to ensure that the coil conductor 19 is not located on a line extending from the axis line of the inner circumferential side via hole conductor 21. FIG. 9 illustrates the insulation layers 13 and 14 illustrated in FIG. 8. Shifting the inner circumferential side via hole conductor 21 to a position in the insulation layer 13 indicated by a broken line, for example, can ensure that the coil conductor 19 formed in the insulation layer 14 thereabove is not located on a line extending from the axis line of the inner circumferential side via hole conductor 21. There is a relatively large open space in the central area of the insulation layer, and thus it is relatively easy to change the position of the inner circumferential side via hole conductor as described above.
On the other hand, with respect to the positional relationship between the outer circumferential side via hole conductor 23 and the coil conductor 20 or the coil conductor 16 mentioned after, it is not easy to ensure that the coil conductor 20 or the coil conductor 16 is not located on a line extending from the axis line of the outer circumferential side via hole conductor 23. FIG. 10 illustrates the insulation layers 14 and 15 illustrated in FIG. 8. To ensure that the coil conductor 20 formed on the insulation layer 15 is not located on a line extending from the axis line of the outer circumferential side via hole conductor 23, it is necessary to shift the outer circumferential side via hole conductor 23 to one of several positions in the insulation layer 14 indicated by the broken lines. However, shifting the position of the outer circumferential side via hole conductor 23 leads to issues such as interference with an intermediate portion of the coil conductor 19, straddling an edge of the insulation layer 14, and so on. In other words, within the limited surface area of the insulation layer 14, it is not easy to ensure that the coil conductor 20 (or the coil conductor 16) is not located on a line extending from the axis line of the outer circumferential side via hole conductor 23 without reducing the number of turns in the coil.
The problem of reduced breakdown voltage reliability caused by the conductive material diffusion, a drop in thickness of the insulation layers, and so on with respect to the outer circumferential side via hole conductors as described above results in a drop in the degree of freedom with which the shape of the coils in the common mode choke coil can be designed. However, increasing the thickness of the insulation layers in order to increase the breakdown voltage reliability poses an obstacle to the miniaturization of the common mode choke coil.